It is known in the vehicle technology that hybrid transmissions utilize an electric auxiliary pump to guarantee the hydraulic supply or rather oil supply. The auxiliary pump is actuated if the oil supply cannot be covered by means of a main pump on the transmission side because the transmission input rotational speed is too low. It is hereby assumed that the electric auxiliary pump can immediately start the oil supply when switched on. However, it requires that the intake port and the intake duct are filled with oil so that no air and no oil foam are present. Otherwise, unwanted air intake takes place when the auxiliary pump is again switched on, so that the oil supply of the transmission during that time interval is below the necessary level and therefore shift elements, for instance, cannot be activated.
For instance, known from the publication DE 10 2008 040 667 A1 is a hydraulic system with a transmission device having a main transmission pump and an auxiliary pump, as well as a method to operate the hydraulic system. In the known hydraulic system, to avoid unwanted outflow of the hydraulic fluid or rather oil, in the area of the auxiliary pump into an unpressurized area or into an oil sump, an additional switch-over valve is required which separates the auxiliary pump from the secondary pressure circuit. This not only requires a larger construction effort but it is also costly.